1. Field of the Invention
The invention relates to a current stabilizing arrangement comprising a first and a second current circuit and a current mirror circuit for sustaining unequal currents which are in a fixed ratio to each other in said current circuits, a first semiconductor element with a main current path and at least a first and a second electrode, of which at least the first electrode is situated in the main current path, the current in said main current path being a defined function of the voltage between said electrodes, of which first semiconductor element the main current path is included in the forward direction in the first current circuit between the current mirror circuit and a first point, a second semiconductor element which is substantially identical to said first semiconductor element and whose main current path is included in the forward direction in the second current circuit between the current mirror circuit and the first point, both semiconductor elements being formed on one substrate, a third circuit between a second point and the first point via the second and the first electrode of the first semiconductor element, a fourth circuit between a third point and the first point via the second and the first electrode of the second semiconductor element, and means for sustaining equal voltages across the third and the fourth circuit.
Said semiconductor elements may inter alia be diodes, the first and the second electrode depending on the forward direction being constituted by anode and cathode, bipolar transistors, the base electrode being the second electrode and the emitter electrode the first electrode, and fieldeffect transistors, the gate electrode being the second electrode and the source electrode the first electrode.
2. Description of the Prior Art
Current stabilisers of the type mentioned in the preamble are inter alia described in the U.S. Pat. No. 3,914,683. In this current stabilising arrangement equal voltages are maintained across the third and the fourth circuit in that the second and the third point are interconnected. These points are each connected to the base electrode of the first and second transistor which constitute the first and the second semiconductor element respectively, whose main current paths are situated in the first and the second current circuit respectively. One of the two transistors may then be connected as a diode by a collector-base interconnection. The fixed ratio of the currents in the two current circuits can then be maintained by a current mirror coupling between the two current circuits or by using a differential amplifier, to whose inputs voltages are applied which are produced across resistors which are included in the first and the second current circuit, an output of said differential amplifier being connected to the ends of said resistors which are remote from the input of the differential amplifier. In the third circuit a resistor is then included between the first semiconductor element and the first point, through which resistor the smaller of the two currents flows.
In a current stabilising arrangement of the type mentioned in the preamble described in "IEEE Journal of Solid State Circuits", vol. SC-8, no. 3, June 1973, pages 222-226 equal voltages are maintained across the third and fourth circuit in that the second and the third point are respectively connected to the inverting and the non-inverting input of a differential amplifier, whose output is connected to a third point. The third point is connected to the second and third point respectively with resistors which are included in the first and the second current circuit respectively. The two semiconductor elements are then diodes or transistors connected as diodes. The ratio of said resistances defined the ratio of the currents which flow through the first and the second current circuit. The third circuit includes a resistor in series with the first semiconductor element, the smaller of the two currents then flowing through this resistor.
Furthermore, a current stabiliser is known from Netherlands Patent Application No. 7,214,136 which has been laid open for public inspection, in which the first and second semiconductor elements are first and second transistors and in which a resistor is included in the second current circuit in the collector circuit of the second transistor, so that said third circuit is established via said resistor and the base-emitter junction of the first transistor and the fourth circuit via the base-emitter junction of the second transistor. The base of the first transistor is then connected to the collector of the second transistor and the base of the second transistor of the end of said resistor which is remote from the collector of the second transistor.
In current stabilisers of the type mentioned in the preamble additional diodes or transistors connected as diodes may be included in third and fourth circuits, provided that equal numbers of these elements are included in both circuits. Furthermore, identical resistors may be added in the third and the fourth circuit.
The operation of current stabilising arrangements of the type mentioned in the preamble is based on the fact that owing to the fixed ratio between the currents in the two current circuits a stable condition can be obtained only for one specific magnitude (unequal to zero) of these currents. Since equal voltages are maintained across the second and the third circuit these currents should meet the requirement that the difference between the voltages between the two electrodes of the second semiconductor element and between the two electrodes of the third semiconductor element must equal the voltage across the resistor included in the third circuit (or, if additional resistors have been included in the two circuits, equal to the difference between the voltages across the resistors in the two circuits).
For the difference between the voltages across two substantially identical semiconductor junctions, which semiconductor junctions in an integrated circuit have virtually the same temperature and are highly identical except for the geometry, it can be demonstrated that this difference is equal to (kT/q) ln (i.sub.02 /i.sub.01), where k is the Boltzmann constant, T the absolute temperature, q the elementary charge, n the ratio of the two currents through the semiconductor junctions, i.sub.01 the reverse saturation current of the one semiconductor junction and i.sub.02 the reverse saturation current of the other semiconductor junction. If the resistor included in the third circuit has a resistance R, the current I through this resistor is then I=(kT/qR) ln n (i.sub.02 /i.sub.01), where i.sub.02 is substantially equal to i.sub.01 because the two semiconductors junctions are substantially identical.
From the foregoing it follows that the currents which flow through the first and the second current circuit have a value which is proportional to the temperature. The current at the first point may then also exhibit the same temperature dependence.
In U.S. Pat. No. 3,914,683 it is stated that by the addition of a resistor of suitable resistance in parallel with the second semiconductor junction, a current which is substantially temperature-independent is available at the first point. This is because the current through this resistor is proportional to the voltage across the second semiconductor junction, through which semiconductor junction a current flows which is proportional to the temperature. For the voltage across such a semiconductor junction it can be demonstrated that this voltage has a temperature-independent component and a component with a negative first-order temperature dependence. The current produced in this resistor by this first-order component may then compensate for the positive temperature dependence of the currents which flow in the two current circuits, so that a substantially temperature independent current is obtained. The two patent applications cited supra also give an example of the voltage equivalent of such a temperature-independent current source. For this the generated current with positive temperature dependence is passed through the series connection of the semiconductor junction and a resistor. The voltage component with a positive temperature dependence which is produced across this resistor by said current can then compensate for the component of the voltage across the semiconductor junction having a negative first order dependence. It can be demonstrated that the voltage across said resistor in series with said semiconductor junction is then substantially equal to Egap, which is the gap between the conduction and the valence bands of the semiconductor material used (in the equivalent current source the current then substantially equals E.sub.gap /R, R being the parallel resistance). In the circuit arrangement in accordance with said article in the "IEEE J.S.S.C" the series connection already forms part of the current stabiliser and the voltage E.sub.gap appears between the output of the differential amplifier and the first common point.
When field-effect transistors are employed similar relationships can be obtained, but in that case square-law instead of exponential characteristics are valid.
In the case of bipolar transistors which have been integrated on one substrate using the same process steps, the equality of the said quantities i.sub.01 and i.sub.02 is mainly determined by the dimensions of the base-emitter junction. Using conventional technologies errors of 1 to 2% occur relative to the desired current (kT/qR) ln n. For applications in which an accurate current or voltage is desired these errors are too great. The error may be reduced by adjustment of the resistance R, but this is undesirable for production purposes. This is even more so in the said applications where a further resistor is included in order to obtain a temperature-independent voltage or current. Both resistors then influence the temperature coefficent and the value of this voltage or current, so that a uniform adjustment is not possible.
When field-effect transistors are used the errors are mainly determined by deviations of the channel dimensions relative to the desired dimensions.